In motor propelled ships it is customary to provide a boiler to supply steam for auxiliary services such as cargo-handling winches and other small steam driven machinery. In most designs of boiler furnaces, a considerable amount of refractory is necessary, and in motor ships this refractory is prone to vibration damage. This damage can be avoided by providing a furnace whose walls, roof and floor are constructed from longitudinally finned tubes welded together to form a gas tight furnace. Such a design, however, involves the use of headers and high drum-centers to ensure adequate circulation under list and rolling conditions. These headers are very expensive and restrict circulation, thus reducing safety margins.
To allow for heat losses and to ensure dry steam at the consumer position, a small degree of superheat is normally provided. But to avoid the penetration of walls in high gas temperature zones and the subsequent complex sealing arrangements, it is necessary to position the superheater outside the boiler and this necessitates having a low-velocity gas flow across the generating tubes with a consequent drop in heat exchange efficiency. Therefore to ensure efficient use of the superheating surface area, it is necessary to accept reduced efficiency of the boiler.
The invention has therefore been made with these points in mind.